Dr. Jeff Ritterman is vice president of the board of directors of the SF Bay Chapter of Physicians for Social Responsibility. He is the retired chief of cardiology at Kaiser Richmond and a former Richmond city councilmember. Follow him on Twitter @JeffRitterman.

On March 20, the International Agency for Research on Cancer (IARC) reclassified glyphosate as a chemical that probably causes cancer. The IARC is a branch of the World Health Organization that focuses on cancer, and it combines the knowledge and expertise of epidemiologists, laboratory scientists, and biostatisticians. The IARC has been engaged in cancer research for more than five decades, and its vast experience in cancer research has led the agency to conclude that "most cancers are, directly or indirectly, linked to environmental factors and thus are preventable."

The IARC had previously designated glyphosate as possibly carcinogenic. Monsanto, a leading producer of glyphosate under the trade name Roundup, immediately issued a press release challenging the new IARC designation and contending that Roundup is safe. But Monsanto has a tremendous amount at stake. Half of the corporation's revenues come from sales of Roundup and Roundup Ready seeds, which can tolerate the herbicide. Monsanto advocates that farmers spray their fields heavily and repeatedly with Roundup in order to kill unwanted weeds, and Monsanto's corporate strategy is based on the assumption that Roundup is safe. If Roundup is found to be toxic, the entire house of cards comes tumbling down, and with it, Monsanto and biotech agriculture. The banning of glyphosate could mean bankruptcy for Monsanto.

But the scientific case for banning glyphosate is convincing. Research shows that in addition to concerns about cancer, there is strong evidence that Roundup causes birth defects in vertebrates, including in humans. The research also reveals that glyphosate may be the cause of or trigger for a number of chronic illnesses that are now plaguing people around the globe.

Originally patented by the Stauffer Chemical Company in 1964, glyphosate is a powerful chelating agent — meaning that it avidly binds to metals. It's this chelating property that led to glyphosate's first use as a descaling agent to clean mineral deposits from pipes in boilers and other hot water systems. The ability to bind to metals also allows glyphosate-metal complexes to persist in soil for decades. The chelating property also underlies the hypothesis that glyphosate-metal complexes are the cause of a fatal chronic kidney disease epidemic that has been ravaging Central America, Sri Lanka, and parts of India.

In the 1970s, John Franz, a Monsanto scientist, discovered glyphosate's usefulness as an herbicide. Monsanto patented glyphosate and has marketed the chemical as "Roundup" since 1974. Glyphosate is now the world's most widely used herbicide.

But contrary to Monsanto's claims that Roundup is safe, a virtual avalanche of scientific studies, including some funded by Monsanto itself, show alarming incidences of fetal deaths and birth defects in animals exposed to glyphosate. Birth defects include missing kidneys and lungs, enlarged hearts, extra ribs, and missing and abnormally formed bones of the limbs, ribs, sternum, spine, and skull.

These startling revelations can be found in the 2011 report "Roundup and Birth Defects: Is the Public Being Kept in the Dark?" It was written by eight experts from the fields of molecular genetics, agro-ecology, toxico-pathology, scientific ethics, ecological agriculture, plant genetics, public health, and cell biology. The report, written primarily for a European readership, is highly critical of the biotech industry and of the European Union's failure to evaluate glyphosate based on science rather than on political concerns. It calls for an immediate withdrawal of Roundup and glyphosate from the European Union until a thorough scientific evaluation can be completed on the herbicide.

"The public has been kept in the dark by industry and regulators about the ability of glyphosate and Roundup to cause malformations," the report states. "In addition, the work of independent scientists who have drawn attention to the herbicide's teratogenic effects has been ignored, denigrated or dismissed. These actions on the part of industry and regulators have endangered public health."

A teratogen is any agent that can disturb the development of an embryo or a fetus. The term stems from the Greek teras, meaning monster.

In late 2012, when Danish pig farmer Ibn Bjorn Pedersen began feeding his pigs genetically modified soy that was contaminated with glyphosate, the rate of birth defects soared. In early 2013, piglets were born without an ear, with only one large eye, with a large hole in the skull, and with a monstrously large "elephant tongue." A female piglet was born with testes, and still others had malformed limbs, spines, skulls, and gastrointestinal tracts. The deformed piglets all tested positive for glyphosate in their tissues.

These birth defects in test animals and in farmer Pedersen's pigs were similar to those reported by humans living in Argentina, where glyphosate is sprayed heavily from airplanes as part of the production of genetically modified soy. In the Córdoba region of Argentina, the Gatica family resides in the barrio of Ituzaingó, only 50 meters away from fields of GMO soy. Airplanes would regularly fly overhead, spraying glyphosate on the crops. In the mid-1990s, Sofia Gatica's oldest son became extremely ill. "When he was four years old, he came down with the illness that left him temporarily paralyzed," she recalled, according to a 2013 report published by the German news organization Deutsche Welle. "He was admitted to the hospital. They told me that they didn't know what was wrong with him."

In 1999, Gatica gave birth to a baby girl. The infant died of kidney failure on her third day of life. This tragedy prompted the grieving mother to take action. Gatica went door-to-door, collecting information on the health of her community. Her survey uncovered an unusually high rate of birth defects and cancer. "Children were being born with deformities, little babies were being born with six fingers, without a jawbone, missing a skull bone, with kidney deformities, without an anus — and a lot of mothers and fathers were developing cancer," she said, according to the Deutsche Welle report.

Gatica shared her findings with her friends and neighbors. Soon a group formed, calling itself the Mothers of Ituzaingó. In 2012, Gatica was awarded the Goldman Environmental Prize for her work protecting her community from glyphosate toxicity.

A group of Argentine doctors, alarmed by the increases in birth defects and cancer, joined the Mothers of Ituzaingó. These concerned physicians formed Doctors of Fumigated Towns, which held its first national conference in August 2010 in Córdoba, Argentina, a farming area where agribusinesses heavily and repeatedly spray glyphosate. The Department of Medical Sciences of the National University at Córdoba sponsored the conference. Some 160 doctors from throughout the country attended. At the conference, Dr. Medardo Avila Vazquez, a pediatrician and environmental health expert, explained his concerns: "The change in how agriculture is produced has brought, frankly, a change in the profile of diseases. We've gone from a pretty healthy population to one with a high rate of cancer, birth defects, and illnesses seldom seen before. There are more than 12 million people affected by fumigation (pesticide spraying) in the country. In these areas, the rate of birth defects is four times higher than in the cities."

Chaco is Argentina's poorest province and a region of intensive glyphosate spraying. Records from the neonatal service at Chaco's Perrando Hospital show that birth defects increased fourfold, from 19.1 to 85.3 per 10,000 people, in the decade after intensive herbicide use began.

The experimental animal studies, the observations in farm animals, and the epidemiological studies in humans all bolster the conclusion that glyphosate causes birth defects.

And the research directly contradicts claims by Monsanto, which states on its website that Roundup is safe "because it binds tightly to most types of soil so it is not available for uptake by roots of nearby plants. It works by disrupting a plant enzyme involved in the production of amino acids that are essential to plant growth. The enzyme, EPSP synthase, is not present in humans or animals, contributing to the low risk to human health from the use of glyphosate according to label directions."

So how can Roundup cause birth defects if it only affects an enzyme (EPSP Synthase) that animals do not possess? Andrés Carrasco, an embryologist and the former director of the molecular embryology laboratory at the University of Buenos Aires, found the link.

Carrasco suspected that glyphosate caused an abnormal hyperactivity in the Vitamin A pathway. The Vitamin A signaling pathway is present in all vertebrates from the very earliest stages of embryonic development. The pathway turns on certain genes and turns off others. It acts like a conductor, orchestrating the symphony of embryological development. And there is no room for error. Genes must be turned on and off at precisely the right instant in exact sequence. Any disturbance of the Vitamin A pathway can result in birth defects. It is because of the enhanced risk of birth defects that pregnant women are advised not to take any Vitamin A (retinoic acid) containing medications.

When Carrasco added a chemical inhibitor to his experiments, he was able to block the glyphosate-induced hyperactivity in the Vitamin A pathway. The birth defects no longer appeared. Mystery solved! Glyphosate had caused birth defects by over-stimulating the Vitamin A pathway. Since this pathway is present in all vertebrates, glyphosate has the capacity to cause birth defects in fish, birds, amphibians, reptiles, and mammals.

But Roundup doesn't just cause birth defects.

Epidemiologic studies from the areas in Latin America where agribusinesses heavily spray glyphosate have consistently shown spikes in cancer incidence. Other epidemiological research has implicated glyphosate in brain cancer in children and in non-Hodgkin's lymphoma. In addition, laboratory studies of many kinds, as well as animal feeding studies, have repeatedly linked glyphosate to cancer.

Cancer is a complex process. One of the initial steps is damage to our DNA. Each of our cells gets its operating instructions from DNA, and if DNA is damaged and not repaired, it can program cells to divide rapidly and chaotically. When that happens, cells transform into cancers.

Cells are also vulnerable to becoming cancers during cell division. Each cell receives from its parent cell an identical copy of DNA. If a mistake occurs during this process, cells receive faulty DNA copies, and the cells can then turn cancerous.

In a 2004 study, researchers from the National Scientific Research Center and the University of Pierre and Marie Curie in France exposed sea urchin embryos to glyphosate, and found that the herbicide caused significant errors in cell division. The scientists commented that these abnormalities are hallmarks of cancer and delivered a particularly chilling warning. "The concentration of glyphosate needed to cause these errors was 500 to 4,000 times lower than the dose to which humans may be exposed by aerial spraying or handling of the herbicide."

Fernando Manas, a biologist at the National University of Rio Cuarto in Argentina, has been investigating the effects of pesticides for years. He believes that glyphosate spraying is causing cancer by inducing DNA damage, and his research has documented genetic damage in those exposed. When Manas studied people who spray pesticide while working in the soy industry in Córdoba, Argentina, he found significantly more DNA damage in their lymphocytes than those in an unexposed group. Glyphosate was one of the most commonly used pesticides by the workers.

Genetics researchers from the Pontifical Catholic University in Quito, Ecuador evaluated Ecuadorians living in the Sucumbíos district in northern Ecuador for evidence of DNA damage. The Colombian government had heavily sprayed the Sucumbíos district with glyphosate to eradicate illegal coca crops. People exposed to the herbicide developed a number of acute symptoms, including abdominal pain, vomiting, diarrhea, fever, heart palpitations, headaches, dizziness, numbness, insomnia, depression, shortness of breath, blurred vision, burning of eyes, blisters and rash. When compared to a control group, they also showed significantly more DNA damage.

In addition to the DNA and cell division research, scientists have explored glyphosate's association with cancer in tissue culture studies. In these experiments, researchers grow cells in a small dish with nutrients and add various chemicals to test their effects.

In 2010, researchers in India exposed mouse skin cells grown in tissue culture to glyphosate. When the herbicide was added, the cells became cancerous.

Scientists in Thailand studied the impact of glyphosate on human estrogen-responsive breast cancer cells in tissue culture. Hormone-responsive breast cancer cells are known to grow when exposed to estrogen. And according to their published results in 2013, glyphosate also stimulated these cells to grow. The herbicide was able to bind to the cancer's estrogen receptors, thus mimicking the effects of estrogen and accelerating tumor growth. Scientists refer to this as "endocrine disruption." An endocrine disruptor is a chemical that can mimic or block a hormone. Because hormones work as chemical messengers at very low doses, even a minute dose of an endocrine disruptor can lead to serious illness.

Glyphosate's links to cancer have also been assessed in studies with a variety of test animals for more than three decades. One of the earliest studies was conducted from 1979 to 1981, under the auspices of the United Nations Environmental Program, the International Labor Organization, and the World Health Organization. Rats exposed to low levels of the herbicide developed testicular cancer. A larger dose did not produce the cancer. Unfortunately, at the time of the experiment, it was not understood that certain substances have more potent effects at lower doses than at higher doses, and so the evaluators erroneously dismissed the results.

In a study from the Institute of Biology at the University of Caen in France, researchers studied glyphosate's effects on rats, and found that glyphosate doubles the incidence of mammary gland tumors. These cancers also developed much faster in rats exposed to glyphosate than in controls. There was also an increase in cancers of the pituitary gland. Originally published in 2012, the report was retracted after the biotech agriculture industry complained. But after extensive review failed to show any fraud or problem with the data, the report was re-published in 2014.

Human epidemiologic studies also have shown a link between glyphosate and cancer.

Argentine physicians working in areas in which glyphosate is heavily sprayed have reported significant increases in cancer incidence. In Sante Fe province, which is an area of intensive herbicide spraying, a house-to-house epidemiological study of 65,000 people found cancer rates two to four times higher than the national average.

Two villages in Chaco province also raised concerns about glyphosate's association with cancer. Researchers compared residents of the heavily sprayed farming village of Avia Terai to people in the non-sprayed ranching village of Charadai. In the farming village, 31 percent of residents had a family member with cancer while only 3 percent of residents in the ranching village had one.

Dr. Avila Vasquez, a doctor working in the heavily sprayed region of Barrio Ituzaingo, noted that cancer was responsible for 33 percent of the deaths in the region, while the cancer death rate in the big cities was only 19 percent.

In addition, scientists from the International Agency for Research on Cancer (IARC), who have analyzed studies spanning almost three decades, have found a positive association between organo-phosphorus herbicides, like glyphosate, and non-Hodgkin's lymphoma. IARC researchers found that the B cell lymphoma sub-type is strongly associated with glyphosate exposure. As mentioned earlier, the IARC published a monograph last month classifying glyphosate as probably carcinogenic.

The most recent research raising concerns about glyphosate's connection to cancer is the linkage to lymphoma. Scientists from the Agency for Toxic Substances and Disease Registry, a branch of the US Department of Health and Human Services, who specialize in illnesses caused by toxic substances, published results of the US Atlantic Coast Childhood Brain Cancer Study in 2009. That study compared children with brain cancer in Florida, New Jersey, New York, and Pennsylvania to age-matched controls. The researchers found that if either parent had been exposed to glyphosate during the two years before the child's birth, the chances of the child developing brain cancer doubled.

Glyphosate's ability to produce birth defects and its association with cancer show that the herbicide actively impacts a number of important biological processes. Scientists have uncovered some of these impacts, and this work may have far-reaching implications for human health.

As noted above, Dr. Carrasco showed that glyphosate causes birth defects in vertebrates by interfering with the Vitamin A signaling pathway. And this pathway is part of a much larger enzyme system known as the "Cytochrome P450" system. This enzyme system is present in most tissues of our bodies. It is an extremely important and complex, responsible for inactivating toxic compounds and metabolizing medications. The Cytochrome P450 system is also important in the metabolism of sex hormones, cholesterol, and Vitamin D. And glyphosate interferes with several of the enzymes in this vital system.

One of the enzymes it inhibits is aromatase, which converts testosterone to estrogen. The testosterone-estrogen balance is fundamental to normal functioning. Glyphosate can mimic estrogen by binding to estrogen receptors, as we saw in the case of glyphosate's ability to accelerate breast cancer cell growth in tissue culture. The herbicide can also prevent the chemical conversion of testosterone to estrogen. Glyphosate's interference with aromatase may explain its association with impaired fertility. Clearly, these endocrine disrupting effects are cause for concern.

Glyphosate is also toxic to many gut bacteria that are important for human health. These bacteria live symbiotically with humans: The human digestive tract provides a friendly environment, full of nutrients for the bacteria, and in exchange, the bacteria perform a number of essential functions, including the synthesis of vitamins and the detoxification of foreign substances. The bacteria also aid immunity and help digestion and the maintenance of the normal permeability of the gastrointestinal tract.

And when glyphosate kills off helpful gut bacteria, other harmful bacteria can proliferate. Studies analyzing the gut bacteria of cows, horses, and poultry have shown that many highly pathogenic bacteria are glyphosate resistant. The loss of helpful bacteria may also make us vulnerable to leaky gut syndrome, ulcerative colitis, and other gastrointestinal maladies.

Research has suggested that the overgrowth of harmful bacteria can also cause a deficiency in essential amino acids and in necessary metals, like zinc and sulfur. The change in bacterial flora may also lead to the overproduction of ammonia.

Because the presence of glyphosate is not tested in our food supply nor by healthcare providers caring for the sick, implicating glyphosate in the etiology of diseases has been difficult. There is concern, however, that a large number of chronic diseases, including neurological illnesses, may be triggered or exacerbated by changes in amino acid, ammonia, and metal concentrations.

The depletion of amino acids, for example, can result in abnormally low levels of the neurotransmitters serotonin and dopamine. Serotonin regulates mood, appetite, and sleep. Its depletion may lead to depression, insomnia, and disorders of the appetite, such as obesity and anorexia. Dopamine depletion in a key brain area is also the hallmark of Parkinson's disease.

Researchers have also found elevated ammonia levels in children with autism. Sulfur deficiency also has been associated with autism and Parkinson's disease, and with Alzheimer's disease and amyotrophic lateral sclerosis. Zinc deficiency, too, has been associated with autism and Alzheimer's disease, and also with attention deficit and hyperactivity disorders.

An interesting finding from a study at the University of Leipzig showed an unexpected association between chronic illness and glyphosate exposure. The researchers tested urine from humans. They found that chronically ill humans have significantly higher glyphosate residues in their urine when compared to healthy people.

Another chronic illness may have a direct link to glyphosate. Peasant farmers exposed to pesticides in Central America, India, and Sri Lanka have developed a new and fatal kidney ailment. The cause has been difficult to pin down. The illness has become known as "Chronic Kidney Disease of Unknown Etiology [CKDu]."

CKDu is now the second-leading cause of death among men in El Salvador. This small Central American nation has the highest kidney disease mortality rate in the world. Neighboring Honduras and Nicaragua also have extremely high rates of death from kidney disease. More men in El Salvador and Nicaragua are dying from CKDu than from HIV/AIDS, diabetes, and leukemia combined. In one area of rural Nicaragua, so many men have died that the community is called "the Island of the Widows."

While the exact cause of the kidney ailment remains under investigation, a leading hypothesis is that glyphosate-metal complexes are to blame. It appears that glyphosate's chelating properties give the chemical the ability to form complexes with heavy metals that can be readily absorbed through the skin, inhaled, or ingested. Scientists are concerned that these glyphosate-metal complexes can travel through the bloodstream to the kidney and destroy the kidney tubule, leading to renal failure and death.

In response, both the governments of El Salvador and Sri Lanka have instituted bans on glyphosate.

Glyphosate and its degradation product amino-methyl-phosphonic acid have been found in air, rain, groundwater, surface water, seawater, and soil. These studies show that glyphosate persists in soil and water for long periods of time. In addition, the amount of glyphosate detected in samples is increasing over time. The chemical is accumulating in our environment. It also accumulates in animal tissue. A study conducted last year at the University of Leipzig showed that cows were excreting glyphosate in their urine. These cows also had comparable levels of the herbicide in their organs (kidney, liver, lung, spleen, muscle, intestine), proving that meat and dairy are a source of glyphosate for humans.

And glyphosate is an essential ingredient in biotech farming. Its residues can be found in a wide variety of food products. Almost all processed food that contains corn (including high fructose corn syrup) or soy has glyphosate contamination. The same is now true for wheat products, because glyphosate has been added to the wheat harvest production method. Meat products derived from animals exposed to glyphosate in their feed will also be glyphosate contaminated.

But because the FDA does not test for glyphosate, we have no way to monitor the damage that the herbicide is wreaking on human health. We know from leaked US State Department cables that support for biotech agriculture is official US policy despite the health risks. It appears that our best chance of protecting our health and that of our children is a grassroots movement to ban glyphosate use.

The data now shows that glyphosate causes birth defects and cancer. There is also good reason to believe that this herbicide causes or exacerbates a large number of chronic illnesses.

There is really no sensible alternative to banning this poison. Two of the world's visionaries have shared their thoughts on this issue.

"Someday we shall look back on this dark era of agriculture and shake our heads," wrote world famous primatologist Jane Goodall in Harvest for Hope: A Guide to Mindful Eating. "How could we have ever believed that it was a good idea to grow our food with poisons?"

The great Indian environmental leader Vandana Shiva added: "We will continue to create the other world that we are sowing, seed-by-seed, inch-by-inch of soil, person-by-person, community-by-community, until all of this planet is embraced in one circle of a resurgent life and a resurgent love."

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